Label filtering for Slack notifications

Currently all Slack notifications for issues go to the same channel. This is not very scalable for larger teams within a project, as frequently a person may only want notifications for a subset of these. For example at GitLab, the ~Monitoring team may want notifications to their channel for just ~Monitoring labelled issues.

This gets more important for other use cases, like those who use monorepos.

Also, there are no options for controlling what issue lifecycle events trigger notifications. (Creation, Modification, Close)

We should improve these options.

Proposal

We should add two filters for defining Slack notifications to a channel.

Which part of the issue lifecycle (creation, modification, closure) to be notified on

Which label(s) are required in order to notify to that channel

Then we could allow multiple of these to be created, to allow flexible routing of notifications.